Aerial and Spaceborne Remote Sensing in African Archaeology: a Review of Current Research and Potential Future Avenues

Aerial and Spaceborne Remote Sensing in African Archaeology: A Review of Current Research and Potential Future Avenues Short Title: Remote Sensing in African Archaeology 1* 1,2 Dylan S. Davis and Kristina Douglass ACCEPTED VERSION 1. Department of Anthropology, The Pennsylvania State University, University Park, PA 16802 2. Institutes of Energy and the Environment, The Pennsylvania State University, University Park, PA 16802 * Corresponding author details: [email protected] Abstract: Africa represents a vast region where remote sensing technologies have been largely uneven in their archaeological applications. With impending climate-related risks such as increased coastal erosion and rising sea levels, coupled with rapid urban development, gaps in our knowledge of the human history of this continent are in jeopardy of becoming permanent. Spaceborne and aerial remote sensing instruments are powerful tools for producing relatively complete records of archaeological settlement patterns and human behavior at landscape scales. These sensors allow for massive amounts of information to be recorded and analyzed in short spans of time and offer an effective means to increase survey areas and the discovery of new cultural deposits. In this paper, we review various case studies throughout Africa dealing with aerial and satellite remote sensing applications to landscape archaeology in order to highlight recent developments and highlight future research avenues. In particular, we argue that (semi)automated remote sensing methods stemming from machine learning developments will prove vital to expanding our knowledgebase of Africa’s archaeological record. This is especially important for coastal and island regions of the continent where climate change threatens the survival of much of the archaeological record. Keywords: remote sensing, landscape archaeology, heritage protection, Africa Résumé : L'Afrique représente une vaste région où les technologies de télédétection ont été largement inégales dans leurs applications archéologiques. Avec les risques imminents liés au changement climatique, tels que la progression de l'érosion côtière et de la montée du niveau de la mer, ainsi que le développement urbain rapide, les lacunes dans nos connaissances sur l'histoire humaine de ce continent risquent de devenir permanentes. Les instruments de télédétection spatiaux et aériens sont des outils puissants pour produire des relevés relativement complets des peuplements archéologiques et du comportement humain à l'échelle du paysage. Ces capteurs permettent d'enregistrer et d'analyser d'énormes quantités d'informations en peu de temps et offrent un moyen efficace d'élargir les zones de prospection et de découvrir de nouveaux dépôts culturels. Dans cet article, nous passons en revue diverses études de cas à travers l'Afrique portant sur les applications de la télédétection aérienne et satellitaire à l'archéologie paysagère afin de souligner les développements récents et les pistes de recherche futures. En particulier, nous soutenons que les méthodes de télédétection (semi-)automatisées issues du développement de l'apprentissage automatique s'avéreront vitales pour élargir notre base de connaissances des archives archéologiques de l'Afrique. Cela est particulièrement important pour les régions côtières et insulaires du continent où les changements climatiques menacent la survie d'une grande partie des vestiges archéologiques. Mots-clés : télédétection, archéologie paysagère, protection du patrimoine, protection du paysage, Afrique Remote sensing instruments are powerful tools for producing relatively complete records of archaeological settlement patterns and human behavior at the landscape scale. Literature on aerial and spaceborne technologies (e.g., satellites, LiDAR, aerial photographs, etc.) in archaeology has demonstrated that multi- and hyper-spectral satellite sensors and aerial platforms such as LiDAR are particularly useful for tackling issues of survey coverage and site identification (e.g., Chase et al. 2012; Lasaponara and Masini 2012; Leisz 2013; Luo et al. 2019; Osicki and Sjogren 2005; Verhoeven 2017). Coupled with machine-learning algorithms, remote sensing offers an effective means to increase survey areas and the discovery of new cultural deposits (Bennett et al. 2014; Davis 2019; Davis et al. 2019b; Trier et al. 2019). Specifically, the use of such technology allows researchers to: 1) investigate large geographic scales in a time efficient (and cost effective) manner; 2) access areas which are difficult to physically visit due to geography, lack of infrastructure, and/or political instability; and 3) achieve enhanced visibility for archaeological survey in environments with dense vegetation or otherwise challenging topography (e.g., LiDAR, SAR). The widespread use of such methods would allow Africanist archaeologists to investigate settlement distributional patterns and landscape use in multiple temporal contexts at extraordinary speeds, as case studies from other areas demonstrate (e.g., Bennett et al., 2014; Davis et al., 2019b; Magnini and Bettineschi 2019). In this paper, we review landscape-scale remote sensing archaeological research conducted throughout the African continent, focusing primarily on the last two decades (Fig. 1) and how these methods can benefit archaeological research in the face of unprecedented climatic shifts and threats to cultural heritage. Specifically, we look at approaches utilizing aerial and spaceborne remote sensing instruments and avenues of research that are yet to be fully utilized in this region. We offer several explanations for why remote sensing has been slow to break into the mainstream of Africanist archaeology. Then, we present examples from Africanist research that illustrate why these methods are essential for protecting and recording the archaeological record in the face of climate change and human impacts. On the African continent, aerial and spaceborne remote sensing approaches have been widely applied, largely utilizing black-and-white aerial photographs to study state formation (Denbow 1979; Evers 1975; Gard and Mauney 1961; Jones 1978; Lampl 1968; Maggs 1976; Mason 1968; Mille 1970; Saumagne 1952; Seddon 1968; Wright 2007). Such studies illustrate the great potential for these approaches to expand our understanding of the archaeological record at the landscape scale and a diversity of social, economic and political processes. But again, these applications have been uneven. Studies by Jones (1978), Maggs (1976), Evers (1975), Mason (1968), and others revolutionized archaeological understanding of Iron Age settlement patterns throughout much of southern Africa. Meanwhile, on African islands, like Madagascar, aerial remote sensing has been much more limited in its archaeological applications (e.g., Fournier 1973; Mille 1970). Since the advent of commercial satellite imagery, only one study (Clark et al. 1998) has been applied in this region. Such insufficient areal coverage of African islands have severely limited our understanding of their settlement history. Neglecting to make use of aerial and spaceborne technologies makes it more likely that African archaeological sites and landscapes will soon be permanently lost. Climate change brings with it threats to archaeological deposits, including coastal erosion and sea-level rise (IPCC 2018; Ministère de l’Environnement, des Eaux, et des Forêts 2006; USAID 2016). Some of the sites most vulnerable to climate change contain the earliest traces of human (and early Homo) history (Erlandson 2012), while others represent the center of ancient global trading networks and are actively eroding (Radimilahy and Crossland 2015). Many coastal and island sites in Africa are also important for understanding past human adaptation and resilience in the face of climate and other pressures (Douglass and Cooper, in press; Thompson and Turck 2011; Turck and Thompson 2016). With today’s impending climate crisis, it is imperative to learn all that we can from these sites before they are lost. Further damage occurs from political instability and conflict (e.g., Casana and Laugier 2017; Francioni and Lenzerini 2006; Harmanşah 2015; Pollock 2016), and economic inequality (e.g., Brodie et al. 2006; Parcak et al. 2016) . To address anthropological questions concerning demography, the nature of social and political organization in prehistory, and the ecological entanglements of early populations, systematic archaeological investigations are required (e.g., Stahl 2005; also see Verhoeven 2017). Remote sensing instruments provide the ability to survey large geographic areas much faster than traditional approaches, as has been demonstrated by many studies throughout the world (e.g., Beck et al. 2007; Bescoby 2006; Bini et al. 2018; Biagetti et al. 2017; Borie et al. 2019; Cerrillo-Cuenca 2017; Casana 2014; Davis et al. 2019b; De Laet et al. 2007; Evans et al. 2013; Freeland et al. 2016; Guyot et al. 2018; Harrower et al. 2013; Jahjah et al. 2007; Johnson and Ouimet 2014; Klehm et al. 2019; Krasinski et al. 2016; Lasaponara et al. 2014; Lipo and Hunt 2005; Meyer et al. 2019; Scheutter et al. 2013; Thabeng et al. 2019; Zanni and Rosa 2019). This ability is vital in the face of accelerated rates of cultural heritage loss, which threatens African communities and livelihoods (Mire 2017). Remote sensing has rapidly advanced over the past several decades, and the application of some of the more recent innovations in image processing appear underutilized within African contexts.

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